The objective of waste disposal sites is to maintain acceptable limits of migration of waste material from the site into other zones including but not limited to groundwater and the atmosphere. There are many mechanisms of waste material migration including but not limited to leaching, surface runoff, gas migration, and biotic uptake. The focus of the present invention and many previous inventions is directed toward controlling leaching.
As used herein, leaching is defined as water flowing through a waste site, dissolving waste constituents, and carrying the dissolved waste constituents beyond the waste site. The amount of leaching that occurs at a particular waste site depends directly on the amount of water flowing through the waste site. Maximum leaching occurs when the site is evenly infiltrated by either a high rate of inflowing atmospheric precipitation such as rain or snow, or by exposure to groundwater flow from an aquifer.
As used herein, waste sites may be characterized as having solid, liquid or a combination of wastes. Solid waste includes but is not limited to municipal refuse, industrial scrap, process wastes, medical waste, nuclear waste, and chemical wastes. Liquid waste includes but is not limited to polluted ponds and lakes.
Most waste sites are solid waste sites located above groundwater aquifers and infiltration of atmospheric precipitation is the main source of water flow through most solid waste sites. Hence, past efforts as well as the present invention are directed toward controlling the amount of leaching due to atmospheric precipitation through solid waste sites.
Past efforts to control leaching tend to fall into four groups, (1) allowing water to flow through a waste site then collecting and disposing the leachate; (2) sealing a waste site and allowing water to flow into the site and accumulate and relying on evaporation to prevent saturation; (3) preventing water from entering a waste site; and (4) preventing water from flowing through a waste site.
Controlling leaching by allowing water flow through a waste site then collecting and disposing the leachate is the subject of several prior patents, of which, the following may be considered representative.
U.S. Pat. No. 3,586,624 to Larson discloses a landfill having an earth sub-base covered with a liquid impervious layer. A first layer of waste is placed on top of the liquid impervious layer and leachate is drained from the landfill and treated. When the first layer of waste is full, it is covered with a liquid impervious layer and the process repeated.
U.S. Pat. No. 4,352,601 to Valiga et al., discloses temporary storage of waste in a bin having a water impervious liner and substantially water impermeable cover for the water impervious liner, and a collection sump outside a perimeter wall of the bin.
U.S. Pat. No. 4,358,221 to Wickberg, teaches a liquid impermeable enclosure with concrete walls. The concrete walls have a plurality of vertical bores filled with sand for the purpose of wicking leachate to gravel filled windrows directing the leachate to a collection pit.
U.S. Pat. No. 4,624,604 to Wagner et al., shows a dual compartment system. The first compartment for receiving waste material is lined with a liquid impermeable flexible liner. The sides of the first compartment are sloped to direct water to a second compartment below the first compartment. The water is treated with a plurality of treatment materials in the second compartment then collected for subsequent treatment and disposal.
German patent 3,445,127 laid open Jun. 19, 1986 granted to Meier discloses a bottom structure for rubbish dumps comprising a catacomb-like structure of arches and columns which support a sealed waste site foundation. The columns or pillars are hollow for collecting leachate that is discharged to a cleaning system.
In addition to methods directed toward leachate collection, German patent 3,502,980 to Anonymous, laid open Jul. 31, 1986, is directed toward complete sealing of the foundation of reservoirs and waste dumps with no provision for removal of leachate. The seal comprises two barrier layers and an intervening permeable layer. In the case of a leak, leachate is collected in the permeable layer and disposed of. The leak is, however, repaired so that normal operation of the foundation is impermeable.
Another approach is to prevent water from entering the waste thereby preventing the formation of leachate. U.S. Pat. No. 4,483,641 to Stoll teaches a means for shedding and evaporating water before it reaches the waste. This is accomplished by covering the waste with a substantially impermeable material such as clay and shaping the clay so that water is directed away from the waste. In addition, below the clay is a ventilation layer comprising graded rock. Open standpipes penetrate the clay to the ventilation layer allowing air to flow through the ventilation layer to evaporate any moisture that seeps through the clay cover.
Yet another approach is to prevent flow through the leachate. U.S. Pat. No. 4,580,925 to Matich et al. discloses a waste landfill having a porous layer surrounding the waste to permit groundwater to flow within the porous layer and reduce the hydraulic gradient across the disposal zone thereby reducing seepage.
U.S. Pat. No. 4,618,284 to Marks is directed toward a waste vault comprising an impervious cover directing water to a gravel filled moat on the perimeter of the waste vault. The clean water in the moat exerts a radial pressure to prevent water within the waste vault from spreading to the surrounding area.
Whether leaching control is achieved by (1) collecting the leachate, (2) accumulating leachate, (3) preventing water entry into a waste site, or (4) preventing water flow through a waste site, most of the methods are applicable to new waste sites, but because the prior methods rely on sealing underneath the waste site, they would be unsuitable for controlling leaching through an existing unsealed waste site. Only one (U.S. Pat. No. 4,618,284) does not rely on sealing beneath the waste site. However, '284 is designed for isolating liquid waste such as a polluted lake or marsh containing a significant amount of water exerting significant hydraulic pressure.
It would be advantageous to have available a method of controlling leachate through a solid waste site that can be applied to an existing waste site without a bottom seal. It would be further advantageous to allow precipitation to flow without needing subsequent collection or disposal.